U.S. patent number 4,288,419 [Application Number 06/137,209] was granted by the patent office on 1981-09-08 for enhanced recovery of sodium carbonate from nacl-containing sodium carbonate solutions.
This patent grant is currently assigned to Intermountain Research & Develop. Corp.. Invention is credited to William C. Copenhafer, Francis Rauh.
United States Patent |
4,288,419 |
Copenhafer , et al. |
September 8, 1981 |
Enhanced recovery of sodium carbonate from NaCl-containing sodium
carbonate solutions
Abstract
A method for enhancing recovery of sodium carbonate from sodium
carbonate solutions which contain sodium chloride. Anhydrous sodium
carbonate is recovered in good yields from aqueous sodium carbonate
solutions containing sodium chloride by evaporative crystallization
at superatmospheric pressure and at a temperature of at least about
120.degree. C.
Inventors: |
Copenhafer; William C.
(Yardley, PA), Rauh; Francis (Plainsboro, NJ) |
Assignee: |
Intermountain Research &
Develop. Corp. (Green River, WY)
|
Family
ID: |
22476285 |
Appl.
No.: |
06/137,209 |
Filed: |
April 4, 1980 |
Current U.S.
Class: |
423/190; 23/302T;
299/5; 423/195; 423/206.2; 423/421 |
Current CPC
Class: |
C01D
7/24 (20130101) |
Current International
Class: |
C01D
7/24 (20060101); C01D 7/00 (20060101); C01D
007/07 (); C01D 007/26 (); E21B 043/28 () |
Field of
Search: |
;23/32T
;423/26T,188,189,190,195,203,205,421,424,426 ;299/5 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Straub; Gary P.
Attorney, Agent or Firm: Egolf; Christopher
Claims
We claim:
1. A method for enhancing recovery of sodium carbonate values from
an NaCl-containing aqueous sodium carbonate solution, which
comprises
subjecting an aqueous sodium carbonate solution which contains at
least about 10% by weight sodium chloride to evaporative
crystallization at superatmospheric pressure and at a temperature
of at least 120.degree. C. to induce crystallization of anhydrous
sodium carbonate crystals without co-crystallization of sodium
chloride until a crystallizer mother liquor containing less than 9%
by weight Na.sub.2 CO.sub.3 and greater than 23% by weight NaCl is
obtained; and
separating the crystallized anhydrous sodium carbonate without NaCl
crystallization from the crystallizer mother liquor to recover an
anhydrous sodium carbonate product, essentially free of sodium
chloride.
2. In the method of producing sodium carbonate from the aqueous
NaOH- and NaCl-containing effluent from a chloralkali electrolytic
diaphragm cell by carbonating the effluent, the improvement for
recovering anhydrous sodium carbonate from the carbonated effluent
which comprises subjecting the Na.sub.2 CO.sub.3 - and
NaCl-containing effluent containing at least 10% by weight sodium
chloride to evaporative crystallization at superatmospheric
pressure and at a temperature of at least 120.degree. C. to induce
crystallization of anhydrous sodium carbonate without
co-crystallization of sodium chloride until a crystallizer mother
liquor containing less than 9% by weight Na.sub.2 CO.sub.3 and
greater than 23% by weight NaCl is obtained; and separating the
crystallized anhydrous sodium carbonate without NaCl
crystallization from the crystallizer mother liquor to recover an
anhydrous sodium carbonate product, essentially free of sodium
chloride.
3. The method of claim 2 which further comprises recycling the
NaCl-containing crystallizer liquor, after recovery of the product
therefrom, to the electrolytic cell.
4. In the cyclic method of recovering alkali values by solution
mining from subterranean trona ore deposits by the steps of
introducing an aqueous sodium hydroxide solvent into contact with
in situ trona ore to effect dissolution of sodium sesquicarbonate
in the ore; withdrawing the resultant sodium carbonate-containing
solution; crystallizing and recovering a sodium carbonate salt from
such solution, leaving behind a mother liquor; causticizing
residual sodium carbonate in the mother liquor; and recycling
causticized mother liquor as aqueous sodium hydroxide solvent,
the improvement for enhancing recovery of sodium carbonate values
from an NaCl-containing aqueous sodium carbonate solution which
results from solution mining trona deposits associated with
substantial deposits of sodium chloride, which comprises subjecting
such aqueous sodium carbonate solution which contains at least
about 10% by weight sodium chloride to evaporative crystallization
at superatmospheric pressure and at a temperature of at least
120.degree. C. to induce crystallization of anhydrous sodium
carbonate crystals without co-crystallization of sodium chloride
until a crystallizer mother liquor containing less than 9% by
weight Na.sub.2 CO.sub.3 and greater than 23% by weight NaCl is
obtained but leaving sufficient sodium carbonate dissolved in
solution for its causticization to sodium hydroxide that is
required in the recycled aqueous sodium hydroxide solvent; and
separating the crystallized anhydrous sodium carbonate without NaCl
crystallization from the crystallizer mother liquor to recover an
anhydrous sodium carbonate product, essentially free of sodium
chloride.
5. The method of claim 1, 2 or 4 wherein the NaCl-containing sodium
carbonate solution subjected to evaporative crystallization
possesses an NaCl:Na.sub.2 CO.sub.3 weight ratio which is less than
2.7:1.
6. The method of claim 1, 2 or 4 wherein the crystallization
temperature is sufficiently high to reduce the sodium carbonate
concentration in the crystallizer mother liquor below 6% by weight
Na.sub.2 CO.sub.3.
7. The method of claim 1, 2 or 4 wherein the temperature of the
crystallizer is at least about 160.degree. C.
8. The method of claim 7 wherein the NaCl-containing sodium
carbonate solution subjected to evaporative crystallization
possesses an NaCl:Na.sub.2 CO.sub.3 weight ratio which is less than
about 4.5:1.
9. The method of claim 1, 2 or 4 which further comprises
introducing an aqueous diluent into the solids-free crystallizer
mother liquor in an amount sufficient to prevent precipitation of
sodium chloride upon cooling of the mother liquor below the
crystallization temperature at atmospheric pressure.
Description
FIELD OF THE INVENTION
This invention relates to a method for recovering anhydrous sodium
carbonate from NaCl-containing aqueous sodium carbonate
solutions.
BACKGROUND OF THE PRIOR ART
Solution mining techniques for recovering soda ash (sodium
carbonate) from subterranean trona ore deposits, such as described
in U.S. Pat. No. 3,184,287 (issued to Gancy), are ordinarily
operated in a cyclic manner. The mining solvent is typically
regenerated from the mother liquor which remains after recovery of
the solid sodium carbonate (monohydrate or anhydrous forms) from
the withdrawn mining solution. High sodium carbonate recoveries are
thus desirable so as to avoid carrying a large inventory of soluble
sodium carbonate in the regenerated recycled mining solvent.
Solution mining of trona ore deposits which are contaminated with
sodium chloride typically results in an appreciable concentration
of sodium chloride in the withdrawn mining solution. This
complicates the recovery of sodium carbonate in high yields because
of the likelihood of co-precipitation of sodium chloride with the
sodium carbonate product.
Prior art methods for crystallizing sodium carbonate from aqueous
sodium carbonate solutions having an appreciable sodium chloride
content do not teach means for recovering high yields of sodium
carbonate, without co-precipitation of sodium chloride. U.S. Pat.
Nos. 2,133,455 (issued to Keene et al) and 3,656,892 (issued to
Bourne et al) disclose that anhydrous sodium carbonate may be
crystallized from sodium carbonate solutions at temperatures around
105.degree.-110.degree. C. via the introduction of an additive like
sodium chloride into the crystallizer. The introduction of sodium
chloride into the crystallizer liquor lowers the
monohydrate-anhydrous transition temperature and thereby yields
anhydrous sodium carbonate, not the monohydrate, as the
crystallized solid.
The method of the present invention provides for enhanced sodium
carbonate recoveries from aqueous sodium carbonate solutions which
also contain sodium chloride, without co-precipitation of salt
which would contaminate the sodium carbonate product.
SUMMARY OF THE INVENTION
In accordance with the present invention, sodium carbonate values
are recovered in enhanced yields from an NaCl-containing aqueous
sodium carbonate solution by subjecting an aqueous sodium carbonate
solution which also contains sodium chloride to evaporative
crystallization at superatmospheric pressure and at a temperature
sufficiently high to induce crystallization of anhydrous sodium
carbonate crystals without co-crystallization of sodium chloride
and yield a crystallizer mother liquor containing less than 10% by
weight Na.sub.2 CO.sub.3 and greater than 22% by weight NaCl, and
separating the crystallized solids from the crystallizer mother
liquor to recover the anhydrous sodium carbonate product.
The crystallization temperature desirably is sufficiently high to
reduce the sodium carbonate concentration in the crystallizer
mother liquor below 9% Na.sub.2 CO.sub.3 and, most preferably,
below 6% Na.sub.2 CO.sub.3. The crystallization temperature is
desirably at least about 120.degree. C., and most preferably at
least about 160.degree. C.
Aqueous NaCl-containing sodium carbonate solutions treated by the
method of this invention may possess fairly substantial amounts of
sodium chloride relative to the sodium carbonate present. In order
to preclude the possibility of co-crystallization of sodium
chloride in the crystallizer, the NaCl:Na.sub.2 CO.sub.3 weight
ratio of the feed solution should be less than about 2.7:1 when the
crystallization temperature is about 120.degree. C. When the
higher, preferred crystallization temperatures are employed,
however, the NaCl-containing sodium carbonate feed solution may
contain a substantially higher proportion of salt: at the preferred
crystallization temperature of about 160.degree. C., the
NaCl:Na.sub.2 CO.sub.3 weight ratio of the feed solution may be as
high as 4.5:1.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic flow diagram that illustrates the prior art
and is included for comparative purposes only, in which a cyclic
solution mining process utilizes a conventional evaporator to
recover anhydrous sodium carbonate from NaCl-contaminated
subterranean trona ore deposits.
FIG. 2 is a schematic flow diagram that illustrates the method of
the present invention, in which an anhydrous sodium carbonate
crystallizer is operated at superatmospheric pressure and a
temperature of 120.degree. C. and is utilized in the cyclic
solution mining process illustrated in FIG. 1 to enhance the soda
ash recovery.
DETAILED DESCRIPTION
The evaporative crystallization is performed, in this invention, at
a sufficiently high temperature to yield anhydrous sodium carbonate
as the solid product, without co-crystallization of sodium chloride
from the Na.sub.2 CO.sub.3 - and NaCl-containing feed solution. The
anhydrous sodium carbonate is desirably crystallized from the feed
solution at a temperature of at least 120.degree. C. Enhanced
yields of anhydrous sodium carbonate are obtained from the
crystallizer feed solution when the crystallization temperature is
preferably at least about 140.degree. C., and most preferably at
least about 160.degree. C.
A superatmospheric pressure is necessary to achieve these
crystallization temperatures. The superatmospheric pressure will
generally range from about 30 psig for a crystallization
temperature of about 120.degree. C. to about 80 psig for a
crystallization temperature of about 160.degree. C.
The evaporative crystallizer is operated, in the invention, in a
manner which yields a crystallizer mother liquor composition
containing less than 10% by weight sodium carbonate. The
corresponding sodium chloride content in such crystallizer mother
liquor is generally in excess of 22% by weight NaCl. The
crystallizer mother liquor composition preferably contains less
than 9% Na.sub.2 CO.sub.3 and, most preferably, less than 6%
Na.sub.2 CO.sub.3. These crystallizer mother liquor compositions
are generally obtained when the crystallizer is operated at
temperatures of 120.degree. C. and 160.degree. C.,
respectively.
Aqueous Na.sub.2 CO.sub.3 - and NaCl-containing solutions which are
suitable for use as crystallizer feed solutions in this invention
are those which contain a substantial amount of sodium chloride.
Particularly suitable are aqueous solutions containing 10% by
weight NaCl or more, up to about 22% NaCl. It should be evident,
however, that solutions containing dilute concentrations of both
Na.sub.2 CO.sub.3 and NaCl may be utilized in this method, being
concentrated either prior to or during the evaporative
crystallization of this invention to yield anhydrous sodium
carbonate without co-crystallization of sodium chloride.
The NaCl-containing aqueous sodium carbonate solutions employed in
this invention may contain a fairly high proportion of sodium
chloride relative to the sodium carbonate content. The weight ratio
of NaCl:Na.sub.2 CO.sub.3 in the crystallizer feed solutions,
however, should be such to ensure that no co-precipitation of
sodium chloride occurs during crystallization of the anhydrous
sodium carbonate product. The maximum permissible NaCl:Na.sub.2
CO.sub.3 weight ratio for the feed solution, above which
co-crystallization of sodium chloride in the anhydrous sodium
carbonate crystallizer cannot be avoided, varies with the
crystallization temperature employed. This maximum NaCl:Na.sub.2
CO.sub.3 weight ratio may be approximated by the following
mathematical expression, in which crystallization temperature is
expressed in degrees Kelvin (.degree.K): ##EQU1##
It should be evident that at the preferred higher crystallization
temperatures, a higher proportion of sodium chloride in the
crystallizer feed solution is tolerable. For example, the
NaCl:Na.sub.2 CO.sub.3 weight ratio in feed solution for a
crystallizer operated at 160.degree. C. may be as high as about
4.6:1 and is preferably less than about 4.5:1. In a crystallizer
operated at 120.degree. C., however, the NaCl:Na.sub.2 CO.sub.3
weight ratio in the feed solution should be less than about 2.9:1
and is desirably less than 2.7:1.
Aqueous Na.sub.2 CO.sub.3 - and NaCl-containing solutions
appropriate for use in the method of this invention may be produced
in cyclic solution mining processes which utilize NaOH-containing
solvents for recovering soda ash from salt-contaminated
subterranean ore deposits. The mining solution removed from the
subterranean mine cavity is treated, via evaporative
crystallization or the like, to precipitate sodium carbonate
(anhydrous or monohydrate forms). The crystallization is not
ordinarily capable of reducing the dissolved sodium carbonate
content in the mother liquor below 10 to 12% by weight Na.sub.2
CO.sub.3.
This Na.sub.2 CO.sub.3 concentration in the crystallizer mother
liquor, however, provides more than the minimal required carbonate
values required to regenerate a suitable sodium
hydroxide-containing mining solvent. Causticization of a 10-12%
Na.sub.2 CO.sub.3 solution yields a mining solvent containing
7.5-9% NaOH. A satisfactory aqueous mining solvent can contain as
little as 3% NaOH, which requires only 4% Na.sub.2 CO.sub.3 in the
liquor being causticized.
Recovery of additional sodium carbonate values from the Na.sub.2
CO.sub.3 - and NaCl-containing crystallizer mother liquor,
preferably reducing it to 4-6% by weight Na.sub.2 CO.sub.3, is
therefore desirable. Sodium carbonate values in the mother liquor
which need not be converted to sodium hydroxide represent an unused
inventory of sodium carbonate product which is continually recycled
in the aqueous mining solvent. Recovery of these sodium carbonate
values thus provides additional valuable product, improving the
overall solution mining process efficiency.
The aqueous sodium carbonate solution may, in addition to its
sodium chloride content, contain other species (e.g., NaHCO.sub.3
unreacted residual NaOH, and the like), which are normally found in
the mining solution recovered from a trona solution mining
operation.
Aqueous sodium carbonate-sodium chloride solutions suitable for use
as crystallizer feed solutions in this invention may also be
derived from chlor-alkali electrolytic diaphragm cell operations,
which ordinarily yield chlorine and caustic soda as products. The
effluent stream from such an electrolytic diaphragm cell is
typically a weak sodium hydroxide solution that also contains
sodium chloride: 10-12% NaOH and 14.5-16% NaCl.
The weak sodium hydroxide solution, rather than being concentrated
to yield caustic soda, may alternatively be carbonated with carbon
dioxide or sodium bicarbonate to yield a sodium carbonate-sodium
chloride solution: 13-16% Na.sub.2 CO.sub.3 and 14-15% NaCl. The
Na.sub.2 CO.sub.3 content of such a carbonated solution is then
recovered, as a solid, NaCl-free sodium carbonate product. The
mother liquor which remains after recovery of the solids ordinarily
contains residual unrecovered sodium carbonate in addition to all
of the sodium chloride.
Recovery of these sodium carbonate values in this mother liquor,
according to the method of this invention, is desirable for several
reasons. An increased yield of sodium carbonate obviously improves
the overall process recovery efficiency, this being a primary
objective.
The mother liquor is ordinarily recycled to the electrolytic cell,
after refortification with additional NaCl, to decompose its sodium
chloride content to chlorine and caustic soda. Residual sodium
carbonate which is returned to the electrolytic cell in the
crystallizer mother liquor is decomposed into sodium hydroxide and
carbon dioxide. The carbon dioxide is an undesirable byproduct
since it complicates liquefaction of the desired chlorine
NaCl-decomposition product.
The anhydrous sodium carbonate that is crystallized in the method
of this invention is recovered from the crystallizer liquor by
conventional liquid/solid separation techniques, e.g., filtration,
centrifugation and the like. The separation and recovery of the
anhydrous sodium carbonate crystals from the crystallizer mother
liquor are desirably performed at near the crystallization
temperature and pressure. This minimizes the possibility of
undesirable redissolving of the anhydrous sodium carbonate product
into the mother liquor and/or co-crystallization of sodium chloride
during separation of the solid product from its mother liquor.
The anhydrous sodium carbonate crystals may be dried, as is, or
converted to sodium carbonate monohydrate by recrystallization from
aqueous solution, and this may also be dried to yield soda ash.
Crystals formed under the preferred crystallization conditions in
this invention have been found to be relatively uniform in quality,
regardless of crystallization temperature.
After recovery of the anhydrous sodium carbonate product crystals
from the crystallizer mother liquor, aqueous diluent, e.g., water,
should be added to the liquor to ensure that no sodium chloride is
precipitated upon cooling of the liquor below the crystallization
temperature at atmospheric pressure. COMPARATIVE EXAMPLE A
In a cyclic solution mining process for recovering soda ash from
NaCl-contaminated subterranean trona ore deposits with an aqueous
mining solvent containing 4.0% by weight NaOH at a temperature of
45.degree. C., the NaCl-containing aqueous sodium carbonate
solution withdrawn from the region of the trona deposit has the
following composition (minor amounts of impurities being
disregarded): 15% Na.sub.2 CO.sub.3, 17.5% NaCl and the balance
water.
Soda ash may be conventionally recovered from this solution as
anhydrous sodium carbonate in an evaporative crystallization
process, such as illustrated in the schematic flow diagram of FIG.
1. The anhydrous sodium carbonate evaporator 100 is operated at a
temperature of 105.degree. C. and at one atmosphere pressure. The
feed solution 10 introduced into the evaporator 100 has the
composition described above.
Sufficient water 12 is removed via evaporation from the anhydrous
sodium carbonate evaporator 100 so as to yield anhydrous sodium
carbonate 14 as the crystalline product and a mother liquor 16
which contains 10% Na.sub.2 CO.sub.3 and 22% NaCl. The mother
liquor 16 is essentially saturated with respect to Na.sub.2
CO.sub.3 and NaCl at the crystallization temperature, 105.degree.
C. Further evaporation of water from the mother liquor at this
temperature is undesirable, since it would result in the
crystallization of both sodium carbonate and sodium chloride from
the solution.
A soda ash production facility, based on the conventional
evaporative crystallization operation described above, represents a
viable scheme for producing one million tons per year of sodium
carbonate. In such a soda ash process 36.9.times.10.sup.5
pounds/hour of mining solution 10 are fed to the anhydrous sodium
carbonate evaporator 100. Evaporative removal of
4.95.times.10.sup.5 pounds/hour of water 12 from this solution in
the evaporator 100 results in 2.60.times.10.sup.5 pounds/hour of
anhydrous sodium carbonate 14 being produced. The evaporator mother
liquor 16, containing 10% Na.sub.2 CO.sub.3 and 22% NaCl, is
produced at a rate of 29.4.times.10.sup.5 pounds/hour.
The evaporator mother liquor 16 is ordinarily causticized, as shown
in FIG. 1, in a causticizer 200 with lime 18 to yield an aqueous
NaCl-containing sodium hydroxide solution 20 and by-product solid
calcium carbonate (not shown in the Figure). Only a portion of the
sodium carbonate in the evaporator mother liquor 16 has to be
causticized to regenerate the required 4.0% NaOH needed in the
recycled aqueous mining solvent 20. The regenerated aqueous sodium
hydroxide solvent 20 thus contains unrecovered sodium carbonate
values which are recycled in this process at a rate of
1.38.times.10.sup.5 pounds/hour of Na.sub.2 CO.sub.3.
Recovery of all of these sodium carbonate values from the recycled
aqueous mining solvent 20 is very desirable since it would increase
the anhydrous sodium carbonate product yield over 50%, from one
million tons/year to 1.527 million tons/year.
EXAMPLE 1
Example 1 illustrates the method of this invention in which an
anhydrous sodium carbonate crystallizer is utilized to recover
additional sodium carbonate values in the solution mining process
described in Comparative Example A. The overall process is shown in
the schematic flow diagram of FIG. 2.
A feed solution 22 from the solution mine operation is introduced
into a conventional evaporator operated at a temperature of
105.degree. C. and at atmospheric pressure. Water 24 is evaporated
to crystallize anhydrous sodium carbonate 26 and yield a saturated
mother liquor 28, as was described in Comparative Example A.
Additional anhydrous sodium carbonate is recovered from the
Na.sub.2 CO.sub.3 - and NaCl-saturated mother liquor 28 in this
process by utilizing an anhydrous crystallizer 400, into which is
introduced the mother liquor 28 from the conventional evaporator
300, as crystallizer 400 feed solution.
The evaporative crystallizer 400 shown in FIG. 2 is operated, in
the method of this invention, at a temperature of 120.degree. C.
and at a superatmospheric pressure of 32 psig. The crystallizer
feed solution 28 contains 10% by weight Na.sub.2 CO.sub.3 and 22%
by weight NaCl, which corresponds to the mother liquor composition
from the evaporator 100 (in FIG. 1) described in Comparative
Example A.
Through the evaporative removal of water 30 from the crystallizer
400, additional sodium carbonate values contained in the feed
solution 28 are recovered as anhydrous sodium carbonate 32 without
co-crystallization of sodium chloride. Sufficient water 30 is
evaporated to yield a mother liquor 34 from the crystallizer 400
containing 9.0% by weight Na.sub.2 CO.sub.3 and 23.0% by weight
NaCl.
The anhydrous sodium carbonate crystallizer 400, operated in this
fashion, recovers an additional 156,900 tons/year of anhydrous
sodium carbonate 32 in addition to the one million tons/year of
product 26 recovered from the conventional evaporator 300. This
additional anhydrous sodium carbonate 32 amounts to about 30% of
the recoverable sodium carbonate values contained in the
regenerated, recycled aqueous mining solvent (stream 20 in FIG. 1)
of Comparative Example A.
It should be noted that the crystallizer 400 could be operated, if
desired, as the sole crystallizer in the process shown in FIG. 2,
with its feed being solution stream 22. The anhydrous sodium
carbonate recovered would then correspond in amount to the total of
streams 26 and 32 in FIG. 2.
After separation of the product 32 of the crystallizer 400 from its
mother liquor 34, the mother liquor 34 is diluted with water (not
shown in the FIG. 1) in order to prevent sodium chloride from
precipitating when the liquor is allowed to cool below the
crystallization temperature. The dilution water added to the mother
liquor 34 serves to compensate for the volume of solution lost in
the underground cavity which replaces the dissolved trona ore.
The crystallizer mother liquor 34 is subsequently causticized, as
shown in FIG. 2, in a causticizer 500 with lime 36 to convert a
portion of its sodium carbonate content to sodium hydroxide, with
calcium carbonate being a solid by-product (not shown in the Fig.).
The causticization is controlled so as to yield an aqueous
NaCl-containing sodium hydroxide solution 38 which contains about
4% by weight NaOH. The NaOH-containing solution 38 is recycled as
regenerated aqueous mining solvent to the region of the
subterranean ore deposit to repeat the cycle of recovery.
EXAMPLE 2
Example 2 is similar to Example 1, except that the anhydrous sodium
carbonate crystallizer 400 shown in FIG. 2 is operated in this
Example at a higher temperature (160.degree. C.) and
superatmospheric pressure (77 psig). At the preferred operating
temperature of this Example, the recovery of additional sodium
carbonate from the Na.sub.2 CO.sub.3 - and NaCl-saturated mother
liquor 28 is improved dramatically, still without
co-crystallization of salt.
The feed to the crystallizer in Example 2 is the same
NaCl-containing sodium carbonate solution utilized in Example 1,
having a composition of 10% Na.sub.2 CO.sub.3 and 22% NaCl (shown
as stream 28 in FIG. 2).
The evaporative crystallizer is operated at a temperature of
160.degree. C. and at a superatmospheric pressure of 77 psig and
yields anhydrous sodium carbonate (corresponding to stream 32 in
FIG. 2) and a crystallizer mother liquor (corresponding to stream
34 in FIG. 2) containing only 6.3% Na.sub.2 CO.sub.3 and 26.0%
NaCl. The crystallization of anhydrous sodium carbonate is
accomplished, moreover, without any co-crystallization of sodium
chloride.
If operated as the crystallizer 400 shown in the conventional soda
ash process of FIG. 2, the anhydrous sodium carbonate crystallizer
of Example 2 increases sodium carbonate yield by over 50%: an
additional 527,000 tons/year of anhydrous sodium carbonate
(corresponding to stream 32 in FIG. 2) are recovered beyond the one
million tons/year of material (stream 26 in FIG. 2) produced from
the conventional evaporator 300 in FIG. 2.
This additional anhydrous sodium carbonate represents virtually all
of the recoverable sodium carbonate values contained in the
solution fed to the crystallizer (block 400 in FIG. 2). The 6.3%
Na.sub.2 CO.sub.3 contained in the mother liquor after this
cystallization is unrecoverable, from a practical standpoint, since
it is fully utilized during causticization to regenerate aqueous
mining solvent.
After causticization of the mother liquor, the NaOH-containing
aqueous mining solvent (corresponding to stream 38 in FIG. 2)
contains minimal sodium carbonate values and has the requisite 4%
by weight NaOH content. The regenerated aqueous mining solvent is
returned to the solution mining and the cycle of recovery is
repeated.
COMPARATIVE EXAMPLE B
In the production of chlorine and caustic soda via electrolysis in
a diaphragm cell, the weak caustic soda solution which is removed
from the diaphragm cell typically contains 11.6% NaOH, 15.9% NaCl
and the balance, H.sub.2 O. This solution may be carbonated with
CO.sub.2 to form an aqueous NaCl-containing sodium carbonate
solution which contains 14.5% Na.sub.2 CO.sub.3 and 15.0% NaCl.
Soda ash may be conventionally recovered from such a solution as
anhydrous sodium carbonate by introducing the solution into an
evaporative crystallizer operated at a temperature of 105.degree.
C. and at atmospheric pressure. Evaporative removal of water from
the anhydrous sodium carbonate crystallizer results in the
formation of anhydrous sodium carbonate as a solid product and a
crystallizer mother liquor which typically contains 10% Na.sub.2
CO.sub.3 and 22% NaCl. For each 100 pounds of solution fed to the
anhydrous evaporative crystallizer, about 7.6 pounds anhydrous
sodium carbonate are recovered and 67.9 pounds of mother liquor are
produced.
Only slightly more than half of the sodium carbonate content in the
crystallizer feed solution is thus recovered, leaving about 47% of
the sodium carbonate unrecovered in the crystallizer mother liquor.
Recovery of the sodium carbonate content, or at least a portion
thereof, in the mother liquor from the crystallizer is clearly
desirable.
If the crystallizer mother liquor stream is simply recycled to the
electrolytic cell, these residual sodium carbonate values represent
an unrecovered inventory which decreases overall process efficiency
and which may contribute to operating problems in the cell, via
CO.sub.2 generated by Na.sub.2 CO.sub.3 decomposition.
EXAMPLE 3
Example 3 illustrates the operation of an anhydrous sodium
carbonate crystallizer operated at a temperature of 160.degree. C.
and a superatmospheric pressure of 77 psig in the method of this
invention. The feed to the crystallizer in this Example is the
NaCl-containing sodium carbonate solution which was derived from
the carbonation of the chlor-alkali electrolytic diaphragm cell
effluent described in Comparative Example B and thus has a
composition of 14.4% Na.sub.2 CO.sub.3 and 15.0% NaCl.
The anhydrous sodium carbonate crystallizer, operated at a
temperature of 160.degree. C. and superatmospheric pressure of 77
psig, recovers about three quarters (75.2%) of the sodium carbonate
values contained in the feed liquor and yields a crystallizer
mother liquor containing a high proportion of salt compared to the
residual sodium carbonate: 6.3% Na.sub.2 CO.sub.3 and 26.3%
NaCl.
By operating the anhydrous sodium carbonate crystallizer in the
matter of this invention, rather than as a conventional anhydrous
crystallizer at a temperature of 105.degree. C. as described in
Comparative Example B, the recovery of NaCl-free sodium carbonate
from the crystallizer feed is significantly improved. For each 100
pounds of solution fed to the anhydrous evaporative crystallizer in
this Example, about 10.9 pounds of NaCl-free anhydrous sodium
carbonate are recovered (vs. 7.6 pounds in Comparative Example B)
and 56.8 pounds of mother liquor are produced. The recovery of
sodium carbonate is thus improved by 42% when compared to the yield
obtained in Comparative Example B. The amount of carbon dioxide
contained in the recycled crystallizer mother liquor of this
Example was thus decreased by over 47% when compared to the
crystallizer mother liquor being recycled in Comparative Example
B.
The crystallizer mother liquor obtained in the method of this
Example, containing only 6.3% Na.sub.2 CO.sub.3, may be recycled to
the chlor-alkali electrolytic cell for conversion of its sodium
chloride content to additional chlorine and caustic soda.
* * * * *